The present invention relates to supported metal catalysts and more particularly to high surface areas coatings which are deposited on a solid support and onto which a metal catalyst is dispersed.
Conventional catalyst supports for fixed bed reactors and the like have comprised a support substrate having a relatively low surface area, such as a ceramic substrate, onto which is deposited a coating having a relatively high surface area. The coating increases the available area upon which the metal catalyst may be dispersed, and therefore increases the volume and mass specific activities of the supported catalyst. In many cases this increased activity is critical to achieving a commercially viable reaction process.
A problem with conventional coatings (hereinafter referred to as “washcoats”) is that they can have a tendency to flake off during use due to flow of fluids through the reactor bed. This flaking can be particularly pronounced in processes in which the supported catalysts are subjected to thermal cycling, due to the difference between the thermal coefficient of expansion of the substrate and the coating. This effect can be reduced by using a silica-based washcoat with a ceramic, silica, alumina, alumina-silica, or other substrate having a thermal coefficient of expansion that is matched to the silica-based washcoat. However, it is difficult to form thick coatings on substrates using conventional silica washcoats. It has also been difficult to achieve sufficient cohesive strength between successively applied coatings and sufficient adhesive strength between the coating and the substrate to withstand the effects of fluids flowing around the supported catalysts using these conventional silica washcoats.
Supported metal catalysts are used in a variety of catalytic processes, including alkylation, ammination, oxidation, hydroformylation, and others. A particular application for supported metal catalysts requiring a catalyst of high durability is in the hydrogenation of edible animal and vegetable oils. Edible oils consist essentially of triglycerides with smaller proportions of mono- and di-glycerides, i.e., esters of the trihydric alcohol glycerol with long chain fatty acids. Triglycerides are represented by the general formula CH2R1—CHR2—CH2R3, where R1, R2 and R3 are the same or different long chain fatty acids. These long chain fatty acid moieties generally contain 3, 2, 1 or 0 carbon-carbon double bonds. In order to increase the storage, oxidative and thermal stability of edible oils, the number of unsaturated carbon-carbon double bonds in the fatty acid moieties is decreased by catalytic hydrogenation. It is most desirable to eliminate or substantially reduce the concentration of fatty acid moieties having three carbon-carbon double bonds (e.g., linolenates), while avoiding hydrogenation of fatty acid moieties having two or one carbon-carbon double bonds (linoleinates and oleinates, respectively). Further, it is desirable that cis-trans isomerization of linolenates and other fatty acid moieties having two double bonds be avoided during the reaction process.
Hydrogenation of edible oils can be accomplished either in a slurry phase with a powder catalyst or in a fixed bed with a formed catalyst. The normal catalyst of choice utilizes reduced nickel as the catalytic species. However, nickel, and especially any nickel oxide, nickel hydroxide, or nickel carbonate present in the catalyst tends to react with the fatty acids to form nickel soaps. These soaps can redeposit on the catalyst or can be removed from the catalyst and accumulate in the slurry phase or can be carried off in the fatty acid liquid in a fixed bed reaction. As the amount of soap deposited on the catalyst increases, the activity of the catalyst decreases. In addition, any dissolved nickel soaps which are carried into the product can be deleterious to the quality of the product. Further, nickel catalysts are inherently toxic.
Some of the problems encountered in the hydrogenation of edible oils can be addressed through the use of structured catalysts, i.e., catalysts provided as monolithic structures comprising through-channels or other open internal void spaces bounded by internal surfaces composed of or supporting a catalyst, through which oils to be processed may flow. However, conventional structured catalysts, particularly structured catalysts provided with washcoats for supported metal catalysts, do not exhibit sufficient cohesive layering on the substrate and/or adhesion to the substrate to survive commercial processing conditions. Thus a need remains for processes for the catalytic hydrogenation of edible oils based on the use of durable structured catalysts for carrying out the hydrogenation process.